There is something strange about those switching transistors. I just looked in more detail in the 2232 manual. There the base resistors are 33 Ohm (!) but still 0.2 W (!). Same transistors, 151-0852-00 or SJE6447. At internet I only read NPN 100 V / 6A bipolar transistors, but could these be Darlington transistors? BTW these are currently listed at ebay from a parted-out 2232.
I realize my impression was just "at first sight".
Albert
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--- In TekScopes@..., "Francis" wrote:
The switching transistors are rather cold than anything else.
I put an heatsink on each of them for the peace of my mind....
You can put your fingers (be careful!) anywhere with no feeling
of excessive heat.
Please take into consideration that the excess of base current
increases the dissipation as well. The FET has roughly 3V VDS, 0.3/0.4 Amps come from the command, then ~1W is dissipated in it.
A 0.3 IC/IB ratio is too much. The only benefit is a lower "on"
switching time, which is useless here due to the softness of the
waweform edges in the transformers. The cons are a higher "off"
time. Since the inverter uses a seperate transformer, drived by
the main transformer for the base command, this can cause troubles.
I have a bag of TIP31. All tested at 250v DC. None failed or
even avalanched. They are Motorola, rated at 40v VCE. Stunning, no?
Maybe Tek sorted their "specials" this way? Who knows...
--- In TekScopes@..., "Albert" wrote:
Could it be that after your modification the switching transistors get hotter? At first sight it is a trade-off between saturation voltage of the switching transistors and voltage across the FET. The sum of these voltages must remain more or less the same, in order to have the same voltage swing at the primaries of the inverter power transformer.
I only have experience with 7000 SPMSs. Good to know that these "modern" supplies are so different, in case my 2232 breaks down...
BTW the manual says those small resistors are 0.2 W types. In for instance a 7704A supply Ic/Ib = 4 but the collector currents are smaller. The resistors are 0.5 W.
Albert
